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IPlanar solid oxide fuel cell stacks are usually assembled using glass-ceramic sealants. Among several requirements, the sealing material needs to be gas-tight and electrically insulating, having a thermal expansion compatible with the other components, sufficient strength of the bond and an adequate viscosity for the joining process. For a successful joining the flow and spreading behavior of a sealant needs to be known and controlled. This study focuses on the effect of reinforcement by filler materials, e.g. yttria-stabilized zirconia fibres or particles, and silver particles in a glass matrix based on the system BaO-CaO-SiO2. In order to evaluate the influence of filler addition on the flow behavior of a glass-ceramic matrix, hot stage microscopy (HSM) was used as method for analysis. This method was already used successfully by other research groups [1 -3] to determine the force-flow characteristics of glass powders in terms of effective viscosities. Our results have proven also that this technique is helpful for the viscosity characterization of glass-ceramics composites. By correlation of standard viscosity data of a reference material known by literature and reproduction of the fix points by HSM experimental data, it was possible to determine the viscosity of the analyzed composite mixtures. Viscosity curves were plotted using the VFT equation. The HSM tests helped choosing the composites with a more favourable viscous behaviour during the joining process; it was possible to adjust viscosity by the addition of YSZ particles and fibers. [ABSTRACT FROM AUTHOR]